In operation of a four-cycle engine, gases produced in the combustion chamber forced under the increased pressure in the combustion chamber to flow through the gap between the piston and the cylinder wall into the crankcase. Additionally, in reciprocating piston engines, the compression of the crankcase by the piston downstroke becomes increasingly important with fewer numbers of cylinders, the greatest effect being in a single cylinder engine. These effects, if unattended, can cause a substantial rise in pressure in the crankcase causing leakage of engine oil. Therefore, it is customary in a four-cycle engine to provide a breather system for releasing the gas pressure.
In a typical breather system, the oil mist in the crankcase is subjected to positive pressure when the piston travels in a downstroke, and the oil mist is forced through a breather passage containing a check or breather valve to an oil separation chamber where the oil in the mist is separated from the gas. On the upstroke of the piston, the pressure in the crankcase changes from positive to negative and the separated oil is drawn back to the crankcase through a return passage by the pressure differential.
To avoid air pollution it is the usual practice to discharge the gas from the oil separating chamber to the air cleaner of the engine, as opposed to releasing the gas to the atmosphere.
In one known type of breather system, a pair of separate covers are attached to the cylinder head and define the rocker arm chamber and the oil separating chamber and a breather valve is mounted in an opening in the inner cover that defines the rocker arm chamber. However, with this type of rocker arm box cover, it is necessary to seal the inner cover to the cylinder head through a gasket and bolt arrangement, and similarly the outer cover is sealed by a gasket and bolts to the inner cover. Due to the use of the double gasket and bolt arrangements, the assembly of the covers to the cylinder head is a time-consuming task which is a substantial contribution to the overall labor cost in assembly of the engine.